1. Field of the Invention
The present invention relates to an intermediate transfer device that transfers a toner image from an image bearing member onto a recording medium using an intermediate transfer member, and to an image forming apparatus using the intermediate transfer device.
2. Description of the Background
In a typical image forming apparatus, single-color toner images of different colors are sequentially formed on an image bearing member and sequentially transferred onto an intermediate transfer member so that a full-color toner image (hereinafter “toner image”) in which the single-color toner images are superimposed on one another is formed. The toner image is then transferred from the intermediate transfer member onto a recording medium, and the recording medium having the toner image there on is conveyed to a fixing device so that the toner image is melted onto the recording medium.
Alternatively, the toner image on the intermediate transfer member may be simultaneously transferred and fixed onto a recording medium at a nip formed between the intermediate transfer member and a pressing member.
Typically, when a toner image is fixed on a recording medium such as paper by application of heat and pressure, the toner image deforms along microscopic concavities and convexities formed on the surface of the recording medium due to the presence of fibers.
The degree of deformation of the toner image varies by location on the surface, and therefore, a toner image fixed on a rough-surfaced recording medium can appear grainy. In particular, a toner image formed on a concave portion deforms much less when fixed thereon because the toner image is not in contact with a pressing member or a pressing force applied to the toner image from the pressing member is small. Therefore, there is a difference in surface texture between toner images fixed on convex portions and concave portions, resulting in uneven gloss in the fixed toner image.
However, a problem is that a toner image on an intermediate transfer member is heated and compressed from a toner image side. Typically, surface layers of the heating members (e.g., heating roller, integrating means) and the intermediate transfer members include a release material to facilitate reliable separation of a toner image from the heating member. As the release material, fluorocarbon resins such as PFA (tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer) and PTFA (tetrafluoroethylene copolymer) are typically used.
On the other hand, when a toner image is heated by the heating member in contact with the toner image, toner particles on the heating member side may have higher temperature than those on the recording medium side. Therefore, the toner particles on the heating member side may be easily melted and deformed. As a result, the melted toner particles tend to adhere to the heating member, thereby causing an offset problem in which the adhered toner particles are retransferred onto the recording medium. It is difficult to prevent the occurrence of the offset problem even if the type (i.e., surface energy) of releasing material is varied.
In addition, fixing performance largely depends on the nature of the surface of the recording medium. Specifically, when the recording medium has a rough surface, the toner image needs to be heated to have a low viscosity or a pressure applied to the recording medium at a transfixing area needs to be increased, so that the toner image anchors in the fibers of the recording medium.
To reduce viscosity, the toner image needs to be heated to a higher temperature with concern of the occurrence of the offset problem. As the temperature of the toner image increases, that of the intermediate transfer member also increases. Further, the heat may transfer to an image bearing member and increase the temperature thereof.
As a result, the temperatures of other devices and members provided adjacent to the image bearing member also increase. Consequently, toner particles contained in a developing device or a cleaning device may be aggregated on or adhered to the device, which is undesirable.
Moreover, when a greater pressure is applied at the transfixing area, components constituting the transfixing area and the intermediate transfer member need to be very durable, resulting in cost increase.
To overcome the problems described above, JP-A-2001-13798 discloses an image forming apparatus in which a toner image on an intermediate transfer member is heated from an intermediate transfer member side.
The image forming apparatus may further include a deformation member to deform the toner image on the intermediate transfer member from a toner image side before being transferred onto the recording medium. When the intermediate transfer member is heated to a higher temperature than the deformation member, the toner image may strongly adhere to the intermediate transfer member, thereby preventing toner particles from adhering to the deformation member.
However, it is difficult to keep the temperature of the deformation member lower than that of the intermediate transfer member constantly. Therefore, toner particles adhered to the deformation member may be retransferred onto the recording medium, resulting in production of abnormal images.